/*
 * Copyright (C) 2012-2017 DataStax Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.datastax.driver.core;

import com.datastax.driver.core.exceptions.DriverInternalError;
import com.google.common.collect.BiMap;
import com.google.common.collect.Maps;
import com.google.common.reflect.TypeToken;
import com.google.common.util.concurrent.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Map;
import java.util.concurrent.Executor;

/**
 * A compatibility layer to support a wide range of Guava versions.
 * <p>
 * The driver is compatible with Guava 16.0.1 or higher, but Guava 20 introduced incompatible breaking changes in its
 * API, that could in turn be breaking for legacy driver clients if we simply upgraded our dependency. We don't want to
 * increment our major version "just" for Guava (we have other changes planned).
 * <p>
 * Therefore we depend on Guava 19, which has both the deprecated and the new APIs, and detect the actual version at
 * runtime in order to call the relevant methods.
 * <p>
 * This is a hack, and might not work with subsequent Guava releases; the real fix is to stop exposing Guava in our
 * public API. We'll address that in version 4 of the driver.
 */
@SuppressWarnings("deprecation")
/**
 * 包装了guva的一些基本用法，目的是兼容不同的guva版本，因为有可能与应用的guva版本冲突。
 */
public abstract class GuavaCompatibility {

    private static final Logger logger = LoggerFactory.getLogger(GuavaCompatibility.class);

    /**
     * The unique instance of this class, that is compatible with the Guava version found in the classpath.
     */
    public static final GuavaCompatibility INSTANCE = selectImplementation();

    /**
     * Force the initialization of the class. This should be called early to ensure a fast failure if an incompatible
     * version of Guava is in the classpath (the driver code calls it when loading the {@link Cluster} class).
     */
    public static void init() {
        // nothing to do, we just want the static initializers to run
    }

    /**
     * Returns a {@code Future} whose result is taken from the given primary
     * {@code input} or, if the primary input fails, from the {@code Future}
     * provided by the {@code fallback}.
     *
     * @see Futures#withFallback(ListenableFuture, FutureFallback)
     * @see Futures#catchingAsync(ListenableFuture, Class, AsyncFunction)
     */
    public abstract <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                         AsyncFunction<Throwable, V> fallback);

    /**
     * Returns a {@code Future} whose result is taken from the given primary
     * {@code input} or, if the primary input fails, from the {@code Future}
     * provided by the {@code fallback}.
     *
     * @see Futures#withFallback(ListenableFuture, FutureFallback, Executor)
     * @see Futures#catchingAsync(ListenableFuture, Class, AsyncFunction, Executor)
     */
    public abstract <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                         AsyncFunction<Throwable, V> fallback, Executor executor);

    /**
     * Returns a new {@code ListenableFuture} whose result is asynchronously
     * derived from the result of the given {@code Future}. More precisely, the
     * returned {@code Future} takes its result from a {@code Future} produced by
     * applying the given {@code AsyncFunction} to the result of the original
     * {@code Future}.
     *
     * @see Futures#transform(ListenableFuture, AsyncFunction)
     * @see Futures#transformAsync(ListenableFuture, AsyncFunction)
     */
    public abstract <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input,
                                                              AsyncFunction<? super I, ? extends O> function);

    /**
     * Returns a new {@code ListenableFuture} whose result is asynchronously
     * derived from the result of the given {@code Future}. More precisely, the
     * returned {@code Future} takes its result from a {@code Future} produced by
     * applying the given {@code AsyncFunction} to the result of the original
     * {@code Future}.
     *
     * @see Futures#transform(ListenableFuture, AsyncFunction, Executor)
     * @see Futures#transformAsync(ListenableFuture, AsyncFunction, Executor)
     */
    public abstract <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input,
                                                              AsyncFunction<? super I, ? extends O> function,
                                                              Executor executor);

    /**
     * Returns true if {@code target} is a supertype of {@code argument}. "Supertype" is defined
     * according to <a href="http://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.5.1"
     * >the rules for type arguments</a> introduced with Java generics.
     *
     * @see TypeToken#isAssignableFrom(Type)
     * @see TypeToken#isSupertypeOf(Type)
     */
    public abstract boolean isSupertypeOf(TypeToken<?> target, TypeToken<?> argument);

    /**
     * Returns an {@link Executor} that runs each task in the thread that invokes
     * {@link Executor#execute execute}, as in {@link java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy}.
     *
     * @see MoreExecutors#sameThreadExecutor()
     * @see MoreExecutors#directExecutor()
     */
    public abstract Executor sameThreadExecutor();

    private static GuavaCompatibility selectImplementation() {
        if (isGuava_19_0_OrHigher()) {
            logger.info("Detected Guava >= 19 in the classpath, using modern compatibility layer");
            return new Version19OrHigher();
        } else if (isGuava_16_0_1_OrHigher()) {
            logger.info("Detected Guava < 19 in the classpath, using legacy compatibility layer");
            return new Version18OrLower();
        } else {
            throw new DriverInternalError("Detected incompatible version of Guava in the classpath. " +
                    "You need 16.0.1 or higher.");
        }
    }

    private static class Version18OrLower extends GuavaCompatibility {

        @Override
        public <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                    final AsyncFunction<Throwable, V> fallback) {
            return Futures.withFallback(input, new FutureFallback<V>() {
                @Override
                public ListenableFuture<V> create(Throwable t) throws Exception {
                    return fallback.apply(t);
                }
            });
        }

        @Override
        public <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                    final AsyncFunction<Throwable, V> fallback,
                                                    Executor executor) {
            return Futures.withFallback(input, new FutureFallback<V>() {
                @Override
                public ListenableFuture<V> create(Throwable t) throws Exception {
                    return fallback.apply(t);
                }
            }, executor);
        }

        @Override
        public <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function) {
            return Futures.transform(input, function);
        }

        @Override
        public <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function, Executor executor) {
            return Futures.transform(input, function, executor);
        }

        @Override
        public boolean isSupertypeOf(TypeToken<?> target, TypeToken<?> argument) {
            return target.isAssignableFrom(argument);
        }

        @Override
        public Executor sameThreadExecutor() {
            return MoreExecutors.sameThreadExecutor();
        }
    }

    private static class Version19OrHigher extends GuavaCompatibility {

        @Override
        public <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                    AsyncFunction<Throwable, V> fallback) {
            return Futures.catchingAsync(input, Throwable.class, fallback);
        }

        @Override
        public <V> ListenableFuture<V> withFallback(ListenableFuture<? extends V> input,
                                                    AsyncFunction<Throwable, V> fallback, Executor executor) {
            return Futures.catchingAsync(input, Throwable.class, fallback, executor);
        }

        @Override
        public <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function) {
            return Futures.transformAsync(input, function);
        }

        @Override
        public <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function, Executor executor) {
            return Futures.transformAsync(input, function, executor);
        }

        @Override
        public boolean isSupertypeOf(TypeToken<?> target, TypeToken<?> argument) {
            return target.isSupertypeOf(argument);
        }

        @Override
        public Executor sameThreadExecutor() {
            return MoreExecutors.directExecutor();
        }
    }

    private static boolean isGuava_19_0_OrHigher() {
        return methodExists(Futures.class, "transformAsync", ListenableFuture.class, AsyncFunction.class);
    }

    private static boolean isGuava_16_0_1_OrHigher() {
        // Cheap check for < 16.0
        if (!methodExists(Maps.class, "asConverter", BiMap.class)) {
            return false;
        }
        // More elaborate check to filter out 16.0, which has a bug in TypeToken. We need 16.0.1.
        boolean resolved = false;
        TypeToken<Map<String, String>> mapOfString = TypeTokens.mapOf(String.class, String.class);
        Type type = mapOfString.getType();
        if (type instanceof ParameterizedType) {
            ParameterizedType pType = (ParameterizedType) type;
            Type[] types = pType.getActualTypeArguments();
            if (types.length == 2) {
                TypeToken valueType = TypeToken.of(types[1]);
                resolved = valueType.getRawType().equals(String.class);
            }
        }
        if (!resolved) {
            logger.debug("Detected Guava issue #1635 which indicates that version 16.0 is in the classpath");
        }
        return resolved;
    }

    /**
     * 检查某个类是否有某个方法的定义。
     * @param declaringClass
     * @param methodName
     * @param parameterTypes
     * @return
     */
    private static boolean methodExists(Class<?> declaringClass, String methodName, Class<?>... parameterTypes) {
        try {
            declaringClass.getMethod(methodName, parameterTypes);
            return true;
        } catch (Exception e) {
            logger.debug("Error while checking existence of method " +
                    declaringClass.getSimpleName() + "." + methodName, e);
            return false;
        }
    }
}
